In this paper, we demonstrate the benefits of using bursts of picosecond pulses for material micromachining and compare
the results with those obtained when using a nanosecond source with similar pulse energy, pulse width and pulse shape.
The picosecond laser source used for the experiments was delivering 60-ps pulses at a repetition rate of 1.8 GHz,
grouped within arbitrarily-shaped bursts having a width that could be varied from 2.5 to 40 ns. The laser output central
wavelength was at 1064 nm and the output beam M2 value was below 1.15. Micro-milling experiments were performed
on silicon for two levels of energy per burst and with different burst amplitude profiles. We show that the maximum
material removal efficiency and the surface quality can be increased by more than 25% when using bursts of picosecond
pulses with respect to nanosecond pulses with similar energy per pulse. Effect of shaping the burst envelope of the
picosecond laser on the maximum material removal efficiency is also presented.

Pulse bursting is demonstrated using a pulsed MOPA fiber laser at 1064 nm for percussion drilling of stainless steel.
Bursts are configured as fast pulse sequences at tens of MHz, with a temporal envelope in the range from 10 to 420 ns.
Their use rapidly enhances the efficiency of material removal by enhancing pulse energy deposition, and quality
improvement. Results are shown for various pulse conditions by changing number of pulses, spacing and peak powers.
Each pulse burst is collectively triggered and amplified as a single pulse group, at repetition frequencies from singleshot
to hundreds of kHz.

In this paper, we present results on a master-oscillator Yb-doped fiber amplifier with 1 kW cw output power (at
1064nm), and near-diffraction limited beam quality (M2<1.4), with internal quantum efficiency >83%. The final
amplifier stage uses a very high Yb-doped 35-um core LMA fiber, using a new process recipe that virtually eliminates
photo-darkening. As a result, high efficiency, SBS-free operation to 1 kW cw power level is obtained, with a phase
modulation bandwidth of only 450MHz, well below other reported results.
To enable single-frequency cw power scaling to kW levels, we investigate LMA fiber waveguide designs exploiting
gain-discrimination, using partially Yb-doped LMA fiber cores, with various diameters up to 80-um. SBS-free, singlefrequency
(few kHz) operation is demonstrated up to 0.9kW cw power. At the lower cw powers (<200W) neardiffraction
limited beam-quality is obtained, but is observed to deteriorate at higher cw powers. We discuss potential
causes, and present a detailed simulation model of kW large-core fiber-amplifiers, that includes all guided modes, fiber
bend, transverse spatial hole burning, gain-tailoring, mode-scattering, SBS nonlinearity, and various thermal effects. This
model shows good agreement with the observed single-frequency power scaling and beam-quality characteristics.

The output power of fiber optic laser systems has been exponentially increasing in the last years. However, non-linear
effects, and in particular stimulated Raman scattering (SRS), are threatening to seriously limit the development pace in
the near future.
SRS can take place anywhere along the laser system, however it is actually the passive delivery fiber at the end of the
system, the section where SRS is most likely to occur. The common way to combat this problem is to use the so-called
Large Mode Area (LMA) fibers. However, these fibers are expensive and have a multimode nature that will either reduce
the beam quality of the laser output or require a careful excitation of the fundamental mode. Furthermore, the larger the
core area, the more complicated it will be to sustain single-mode operation. Therefore, it is becoming increasingly
important to be able to determine which is the minimum core area required in the delivery fiber to avoid SRS.
This calculation is usually carried out using the conventional formula for the Raman Threshold published by R.G. Smith
in 1972: Pth =16Aeff gRLeff . In this work we demonstrate that this formula and the conclusions derived from it are
inaccurate for short (several meters long) LMA fibers. For example, one widely spread belief (obtained from this
expression) is that there is no dependence of the Raman intensity threshold (Ith=Pth/Aeff) on the mode area. However, our
calculations show otherwise. Additionally, we have obtained an improved Raman threshold formula valid for short LMA
fibers.

We describe a time-dependent model that describes the evolution of stimulated Brillouin scattering (SBS) in fibers under
phase-modulated pump conditions. In order to accurately model fast modulations, the triply-coupled system of
differential equations describing the interaction of SBS through optical and phonon fields is solved numerically. SBS is
initiated from noise by using a Langevin term. We initially consider single-frequency sinusoidal modulations as a
function of modulation amplitude and frequency. We then investigate the effects of SBS mitigation when a single-frequency
seed is phase modulated with a broad-band white-noise source (WNS).

The efficiency of two tone fiber amplifiers can be changed rather significantly by altering the temperature of the external
environment surrounding the gain fiber. It is shown experimentally that changes in the temperature of the core of the
gain fiber has dramatic effects on the 1064 nm / 1040 nm power distribution in the output of narrow linewidth 1064 nm
two tone amplifiers with a greater percentage of the output being 1064 nm at higher core temperatures. By increasing
the environmental temperature of the gain fiber from 20 to 80°C, the efficiency of a 1064 nm two tone amplifier can be
increased up to 40% with the greatest increases seen in amplifiers seeded hardest in 1040 nm, i.e., having the smallest
1064 nm / 1040 nm seed ratios. This has been attributed to temperature dependence of the absorption and emission
cross-sections at the wavelengths of interest. Finally, the temperature of the gain fiber can be used as a design tool to
enable a higher efficiency 1064 nm two tone amplifier.

We present experimental studies of a novel polarization-maintaining (PM) Yb-doped photonic crystal fiber (PCF)
possessing a two-segment transverse acoustic profile and a mode field diameter (MFD) of approximately 30 μm. The
concentrations of the dopants (fluorine, aluminum, germanium) in the two segments were selected such that the
corresponding Brillouin shifts were sufficiently separated to allow for the introduction of a large thermal gradient for
further SBS suppression. A pump-probe experiment was conducted in order to characterize the Brillouin gain spectrum
(BGS) and to confirm the existence of two narrow Brillouin gain peaks. The separation of the two peaks was
approximately 220 MHz and the bandwidth of each was estimated to be 50 MHz. The application of a step temperature
profile resulted in the BGS displaying four distinct peaks; thus demonstrating further SBS suppression through a thermal
gradient. By utilizing the thermal gradient obtained through quantum defect heating, we obtained 494 W of output power
in a counter-pumped configuration without the onset of SBS. Measurements of the beam quality at various power levels
and up to the highest reported power consistently indicated an M2 of less than 1.3.

The power handling capabilities of fiber lasers are limited by the technologies available to fabricate and assemble the key
optical system components. Previous tools for the assembly, tapering, and fusion of fiber laser elements have had
drawbacks with regard to temperature range, alignment capability, assembly flexibility and surface contamination. To
provide expanded capabilities for fiber laser assembly, a wide-area electrical plasma heat source was used in conjunction
with an optimized image analysis method and a flexible alignment system, integrated according to mechatronic
principles. High-resolution imaging and vision-based measurement provided feedback to adjust assembly, fusion, and
tapering process parameters. The system was used to perform assembly steps including dissimilar-fiber splicing,
tapering, bundling, capillary bundling, and fusion of fibers to bulk optic devices up to several mm in diameter. A wide
range of fiber types and diameters were tested, including extremely large diameters and photonic crystal fibers. The
assemblies were evaluated for conformation to optical and mechanical design criteria, such as taper geometry and splice
loss. The completed assemblies met the performance targets and exhibited reduced surface contamination compared to
assemblies prepared on previously existing equipment. The imaging system and image analysis algorithms provided in
situ fiber geometry measurement data that agreed well with external measurement. The ability to adjust operating
parameters dynamically based on imaging was shown to provide substantial performance benefits, particularly in the
tapering of fibers and bundles. The integrated design approach was shown to provide sufficient flexibility to perform all
required operations with a minimum of reconfiguration.

A polarization-maintaining (PM), narrow-linewidth, continuous wave, thulium fiber laser is demonstrated. The laser
cavity is formed from two femtosecond-laser-written fiber Bragg gratings (FBGs) and operates at 2054 nm. The
laser output possesses both narrow spectral width (78 pm) and a high polarization extinction ratio of ~18 dB at 5.24
W of output power. This laser is a unique demonstration of a PM thulium fiber system based on a two FBG cavity
that produces high PER without any free-space elements. Such a narrow linewidth source will be useful for
applications such as spectral beam combining which often employ polarization dependent combining elements.

Recent progress on high efficiency Tm-doped silica fibers pumped at 790nm has enabled the
demonstration of a 2μm CW fiber laser operating at the 1kW power level and with single mode
beam quality [1]. In addition to this state-of-the-art high power research, Tm-doped fibers are now
starting to find applications in lasers with nsec [2] and psec [3] pulsed operation, as well as lower
power CW lasers (50-100W) in the 1940nm wavelength region for medical use [4].
As with Yb-doped fibers, the question of photodarkening needs to be addressed to ensure the long
term fiber reliability is appropriate for the application. In a recent paper [5], we proposed that the
up-conversion process in highly doped Tm-fibers was significantly quenched when compared with
lower concentration fibers, under 790nm pumping. This trend was also observed in the
photodarkening rate as measured in a CW 2μm fiber laser cavity operating around 20W output
power. In this controlled experiment, the rate of photodarkening dropped from 15% per 1000 hours
in low concentration fiber to less than 1% in a fiber doped with 4.6% Tm.
In this paper we review the 20W results of our earlier work and then confirm the long term
reliability of 1940nm CW fiber lasers operating at higher (40W) output power, presenting results for
a laser operating for 1200 hours without significant loss of output power (around 12% total power
variation). Over any given 24-hour period during this experiment, the laser operates open loop with
around 8% total power variation, in an air cooled configuration. We believe these results confirm
the appropriate level of long term reliability of Tm-doped fibers for CW fiber lasers at the ~50W
power level, suitable for medical laser applications.

Photodarkening phenomenon in ytterbium (Yb)-doped silica glasses was experimentally investigated by measurements
such as electron spin resonance (ESR), X-ray absorption fine-structure (XAFS), and optical transmittance. A
predominant increase of Al-oxygen hole center (OHC) was observed for Al-Yb co-doped silica glasses both by the
incidence of pump light and by gamma-ray irradiation. It was also recognized that the optical transmission loss similarly
increased in both cases. These results indicate that the formation of Al-OHC is the prime cause of the excess loss
induced by photodarkening. XAFS measurement indicated that the second-nearest-neighbor atoms around Yb may be
related to the mechanism of photodarkening phenomenon.

Photodarkening-induced output power degradations and long-term stability in high power pulsed and CW fiber laser
MOPA systems are discussed. The studied laser systems are based on aluminosilicate single-mode Yb-doped fibers and
use the GTWave fiber technology for cladding pumping. The active fiber lengths are between 10-30m. We have tested
Yb-doped fiber amplifiers operated under both pulsed and CW mode. Using OTDR background-loss measurements we
show, for the first time, that the photodarkening-induced loss is non-uniformly distributed along the length of the active
fiber. By calculating the average inversion along the fiber length, we show that the induced loss follows closely an Yb-inversion
dependence to the power of 2. In addition we have studied the temperature dependence of the output power
variation. It is shown that increasing (decreasing) operating temperature results in decrease (increase) of the laser output
power, reaching the new equilibria over time scales of ~200hours. We also present data on the non-photodarkening SPI
fiber which is used in all SPI products.

The Tm-doped silicate glass fibre laser that operates in the 2 micron region of the spectrum is fast becoming a mature
technology with output powers already exceeding 1 kW. In this paper, I will review a number of current and future
experiments that involve lasers pumped with the output from Tm-doped silicate glass fibre lasers including linear
systems e.g., the optical excitation of rare earth ions and nonlinear systems e.g., Raman fibre lasers.

We report the simultaneous excitation of multiple Raman Stokes lines in a 250 m long fiber using multi-step pump
pulses. The frequency doubled output of a single polarization all-fiber Yb-doped MOPA operating at 1060 nm was used
as the pump source. By adjusting the pump power and the pulse profiles we achieved the simultaneous excitation of
green (1st Stokes), yellow (4th Stokes) and red light (6th Stokes) using 3-step pulses or the combination of any two using
2-step pulses. Through the use of pulse shaping we generate sequences of colored pulses with the flexibility of providing
dynamic, agile frequency tuning between well-defined wavelengths.

We have developed several high power picosecond sources using rod-type fiber systems. We will show that a
very simple architecture can produce average power over 90 W @ 1030 nm, 57 W @ 515 nm and 20 W @ 343
nm, with pulse repetition rates ranging from 200 kHz to 80 MHz. Particular emphasis will be given on the
control of non-linear effects in the fiber without relying on CPA.

We present second harmonic generation of a high average power, high energy femtosecond Yb doped fiber chirped pulse
amplifier. This system is operated at various repetition rates at a central wavelength of 1040 nm. After two pre-amplification
stages a main amplifier is used to achieve the required pulse energy for efficient second harmonic
generation. It is comprised of a 1.2 m long photonic crystal fiber with a mode field diameter of 45 μm. A dielectric
grating based compressor is used for compression to a pulse duration of about 406 fs. Second harmonic generation is
then achieved in a 500 μm thick BBO crystal. The conversion efficiency of the second harmonic generation remained
almost constant at >60 % for all repetition rates and average power levels. At 5.25 MHz the highest average power of
135 W at 520 nm was achieved. In addition this comes with an excellent beam quality which is validated by a measured
M2 < 1.2.

3C (Chirally-Coupled Core) optical fiber establishes a technological platform for high brightness, power scalable lasers
with an engineerable fiber geometry that enables robustly single-mode performance of large core diameter fibers. Here
we report the demonstration of robust polarization preserving performance of 35 μm core 3C fiber for short pulse
systems. A polarization extinction ratio (PER) of ~ 20 dB is stably maintained with ambient temperatures varying over a
50°C range from a Yb-doped double clad 3C fiber amplifier. We also demonstrate that this high-PER polarization
output is insensitive to temperature gradients and mechanical perturbations in the 3C fiber amplifier. The ability to
deliver high peak power pulses at high average powers while maintaining exceptional beam quality and a stable
polarization state in an easily integrated format makes 3C fiber laser systems extremely attractive for harmonic
generation to visible and UV wavelengths.

It is relatively straightforward to completely measure both long (>10ns) and very short (<100ps)
laser pulses in time. But intermediate pulse lengths-that of the most common laser pulses-
remain nearly immeasurable and, not coincidentally, correspond to the least stable of all lasers.
True, ultrahigh-bandwidth oscilloscopes and streak cameras can now resolve such pulses, but such
exotic electronic devices are expensive and fragile and only yield the temporal intensity and not
the temporal phase. Here we describe a simple, elegant, accurate, complete, compact, all-optical,
entirely passive, and single-shot FROG device that solves the problem. It simultaneously achieves
a very large delay range of ~10ns and very high spectral resolution of <1pm. It accomplishes both
feats using high-efficiency, high-finesse etalons, the first to tilt the pulse by 8&lap;9, .t9hat is, by
several meters over a centimeter beam, and another to generate massive angular dispersion for a
high-resolution spectrometer. We demonstrate this device for measuring pulses 100ps to several
ns long from a fiber-amplified micro-disk laser.

We investigate the feasibility of pulsed fiber amplifier coherent combining. Therefore, we characterize phase fluctuations
in low-peak-power pulsed fiber amplifiers using two different interferometric techniques. These measurements reveal
that for low peak-powers, phase fluctuations remain moderate during the pulses. Noticeable phase fluctuations occurring
between the pulses can be perfectly controlled using classical continuous-wave-efficient combining techniques. Results
of such realization combining two low-peak-power pulsed fiber amplifiers, using classical frequency-tagging coherent
combining techniques, are presented. Phase difference measurement is performed between pulses using a small signal
leak from the common master oscillator. For the first time to our knowledge, successful coherent combining of two low-peak-
power pulsed fiber amplifiers is thereby demonstrated.

Here we report a compact monolithic 2000nm pulsed laser with a single spatial mode output, ~1.5ns pulse duration, 8kW
peak power and >200mW average power at 20 kHz repetition rate. The gain-switched laser, consisting of a pair of fiber
Bragg gratings and 0.5m of thulium-doped single cladding fiber, was core pumped by a high peak power pulsed 1.5μm
laser. When the input pulse energy of the 20 kHz pump pulses was sufficient enough to saturate the Thulium doped fiber,
a stable 20 kHz pulse train was observed with measured linewidth of 0.05nm which corresponds to the limit of resolution
for the Optical Spectrum Analyzer. This compact, pulsed 2000 nm laser, to the authors' limited knowledge, represents
the first Tm-doped fiber laser with 8kW peak power and several ns pulse duration; which is less than the previously
reported tens of ns pulsewidth previously reported from gain-switched Tm-doped fiber lasers.

We report here the successful realization of 25 millions wavelengths per second using an SOA based PL around 1565
nm at a 75 MHz repetition rate. The laser is simply composed of an SOA, a CFBG (10 ps/nm) with a 100 nm bandwidth,
an optical circulator, an EOM (intensity modulator), and an output coupler (20%). Pulse duration is around 45 ps and
OSNR of the pulse is around 35 dB at 1565 nm without sweeping. Tunable dispersion compensating module (TDCM)
was used to compress the chirped pulse output and 10 ps pulse duration was obtained at 1548 nm. Finally 25 megawavelengths
per second was realized with under 3 pulses per wavelength and 1024 discrete wavelengths. Linear k-space
sweeping function was enabled in the swept-source OCT (SS-OCT) system through graphical user interface (GUI).

We present the realization of an actively mode-locked laser based on a 30 μm core diameter single-mode double clad
photonic crystal fiber. For 19 W of pump power at 976 nm, it yields an average power of 10 W at 40 MHz. The delivered
pulses, centered at 1030 nm, have a duration of 15 ps. This corresponds to an energy of 250 nJ per pulse and a peak
power of 17 kW.

We report on dual-stage fibered Master Oscillator Power Amplifier system designed to meet stringent requirements of
large-scale laser facilities front-end sources. The combination of high power continuous wave laser and electro-optical
modulator allows the generation of multi-kHz nanosecond pulses with accurate tailored temporal pulse profiles. The seed
pulses are amplified in a preamplifier to achieve 20 μJ pulse energy (33 dB gain) launched inside the power amplifier.
We finally obtained a strictly single mode beam, narrow-bandwidth, multi-kHz nanosecond pulse amplification at mJ
level, with more than 50 dB optical signal-to-noise ratio, 15 dB polarization extinction ratio and sub-nanosecond
resolution temporal shaping. The choice of optimal parameters and the temporal pulse shaping have been performed
thanks to numerical simulations (including forward and backward amplified spontaneous emission) in excellent
agreement with the results.

Amplification of a gain-switched laser diode is demonstrated in an all-fiber based setup. The amplified spontaneous
emission between two consecutive pulses was investigated quantitatively in the time domain. A maximum pulse energy
of 13 μJ at a repetition rate of 1 MHz and a pulse duration of 40 ps was extracted, corresponding to a peak power of 270
KW. Temporal pulse deformation due to intrapulse Raman scattering was observed.

We report on nonlinear optical compression of passively Q-switched pulses accessing sub-10 ps domain, which is so far
dominated by mode-locked systems. The concept implements the SPM-induced spectral-broadening of passively Q-switched
microchip pulses in optical waveguides and a supplementary compression with bulk optics e.g. a pair of
diffraction gratings or a chirped-Bragg-grating. Used seed-source is a fiber-amplified, passively Q-switched microchip
laser operating on a single longitudinal mode and consists of a monolithically bonded combination of Nd:YVO4-crystal
and semiconductor saturable absorber mirror. The microchip laser provides pulses with durations of 100-150 ps, pulse
energies of ~200 nJ at various repetition rates from hundreds of kilohertz to more than a megahertz, and line width of
~20 pm at wavelength of 1064nm. During the amplification process in the photonics crystal fiber, the pulses are
spectrally broadened to up to ~0.7nm at energy of 17μJ. Using a diffraction grating compressor with 1740 l/mm, the
pulses are compressed to duration as short as 6ps assuming a numerically calculated de-convolution factor of 0.735. To
the best of our knowledge, this is the first reported realization of nonlinear compression of the Q-switched pulses and the
shortest pulses from a passively Q-switched laser system.

The major challenge in the development of monolithic kW class CW fiber lasers is the efficient conversion of pump
photons into a high brightness laser beam under the constraints of heat management, long term stability and
nonlinearities. This article reviews the interaction of some fiber related aspects as e.g. fiber core composition,
photodarkening and modality, as well as their influence on system complexity and power scalability. Recent work on
active fibers, pump couplers, mode field adaptors and other fiber-optic components will be presented.

Progress in advanced specialty fibers is the foundation to further breakthroughs in fiber lasers. Recently, we have been
working to advance several areas of developments in specialty fibers and would like to review these efforts here. The
first topic is in the further development of all-glass large core leakage channel fibers (LCF) for robust and practical
solutions for power scaling. The second area is the development of wide band air-core fibers with an innovative square
lattice cladding and the demonstration of a factor of two improvements in bandgap over conventional hexagonal lattice.
These air-core fibers are critical for fiber delivery solution of both CW and pulsed fiber lasers in the future. The last
topic is a new development in design and simulation of SBS gains in optical fibers by incorporating leaky acoustic
modes. These leaky acoustic modes have been mostly overlooked so far. It is essential that they are considered in SBS
simulations in fibers, because they are normal solutions to the acoustic waveguide equations and have similar loss to
guided acoustic modes where the acoustic mode loss is dominated by material loss. This leads to much improved
resolution of SBS gain spectrum in fibers and to new design insights to the limit of SBS suppression based on anti-guide
acoustic waveguide designs.

Ytterbium-doped Large Pitch Fibers with very large mode areas are investigated in a high power fiber amplifier
configuration. An average output power of 294 W is demonstrated, while maintaining robust single-mode operation with
a mode field diameter of 62 μm. Compared to previous active large mode area designs the threshold of mode instabilities
is increased by a factor of about 3.

We have developed a commercial 4-kW fiber laser consisting of seven, 600-W modules whose outputs are combined
with a fused-fiber combiner. The system architecture has several practical advantages, including pumping with reliable
single-emitter diodes, monolithic fused-fiber construction (no free-space beams), and end pumping using a 91:1 pump
combiner (eliminating the need for complex pump/signal combiners). Typical results at 4-kW output power are a beamparameter
product of 2.6 mm-mrad, 8-hr power stability of < 0.5% rms, central wavelength of 1080 nm, and linewidth of
1.2 nm FWHM. These lasers have been incorporated into Amada machines used for cutting metal sheet and plate and
have been used to cut aluminum, mild steel, stainless steel, brass, titanium, and copper with a thickness up to 19 mm. A
world-record cutting speed of 62 m/min has been demonstrated for 1-mm aluminum sheet metal.

We report on a novel concept to scale the performance of ultra-fast lasers by means of coherent combination. Pulses
from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently
combined after amplification. The splitting and combination process is based on the polarization combining technique
using polarization cubes. A Hansch-Couillaud detector measures the polarization state of the combined beam. The error
signal (deviation from linear polarization) is used to stabilize the optical path lengths in the different channels with a
piezo mounted mirror. In a proof-of-principle experiment the combination of two femtosecond fiber-based amplifiers in
a CPA systems is presented. A combining efficiency as high as 97% has been achieved. Additional measurements were
carried out to investigate the stability of the system. The concept offers a unique scaling potential and can be applied to
all ultrafast amplification schemes independent of the architecture of the gain medium.

Lasers that produce 100 kW level diffraction limited power will require beam combining due to fundamental thermal and
nonlinear limitations on the power of single aperture lasers. Towards this goal, we present high power, high spectral
density beam combining by volume Bragg gratings of five 150 W beams with a spectral separation of 0.25 nm between
beams, the narrowest to date for high power. Within 1 nm, 750 W of total power is combined with greater than 90 %
efficiency. Combined beam quality is discussed including the effect of unequal individual beam divergences on the
combined beam quality. The individual input beams may have unique divergences as they enter the system, and the
heated volume Bragg gratings (VBGs) may introduce very slight changes in divergence to each beam. These small
differences in beam divergence between the beams will not degrade the M2 of the individual beams, but the composite
M2 after combination can be adversely affected if the beams do not have equivalent divergence at the output of the
system. Tolerances on beam divergence variation are analyzed and discussed. High power beams transmitting through
or diffracting from a VBG can experience different distortions resulting from thermal effects induced in the VBGs. Each
beam also experiences a different aberration, as no two beams pass through the same number of identical VBGs. These
effects are studied with experiment compared to modeling. Possible methods of beam quality improvement are
discussed.

We perform sensitivity analyses on two different array configurations of coherently combined fiber amplifiers to study
the impact of residual phase errors onto the combining efficiency. The arrays studied are: a square of 16 fibers and a
hexagon of 19 fibers. For the hexagon, two different shaped wavefronts are studied. In this method a global analysisis
performed: we modify simultaneously all the phase errors using numerical space filling designs. Then, the construction
of a metamodel makes it possible to investigate more precisely the role of each fiber and specially the role of interactions
between fibers onto the combination with less runs than classical approaches. Results exhibit different behaviors and
specially interactions between fibers with respect to the array configurations and with respect to the two different shaped
wavefronts. Finally, we demonstrate that we can study arrays of more than 100 fibers.

We demonstrate experimentally that few atomic layer graphene possesses ultrafast, super broadband
saturable absorption, which can be exploited to passive mode lock fiber lasers of different operation
wavelengths. Passive mode locking performance of an erbium-doped fiber laser operating at the
1.55μm and an ytterbium-doped fiber laser operating at the 1.06μm with a graphene saturable absorber
mirror has been experimentally demonstrated, and wavelength tuneable solitons, vector solitons, as well
as the bound state of solitons have been observed.

An Yb-based 78-MHz repetition rate fiber-amplified frequency comb is used to investigate the power scaling
limitations of a standard-design bow tie high-finesse enhancement cavity for XUV generation. With a Xenon
gas jet in the 22-μm-radius focus, the 200-fs intra-cavity circulating pulse reaches a maximum of 20 kW of time-averaged
power. A novel cavity design is presented, conceived to overcome the observed enhancement limitations
and offering the prospect of few-nm high-power high-harmonic generation. Several applications which come into
reach for the first time are discussed.

In this contribution, we report the generation of high energy, high temporal quality, ultrashort
pulses from a single stage double-pass short length Ytterbium-doped rod type fibre chirped pulse
amplification setup. The fibre used as gain medium is a 95-cm long rod type microstructured fibre with a
80 μm core diameter and 200 μm pump cladding diameter seeded by 3 nJ pulses at 100 kHz of repetition
rate. We demonstrate amplification and compression up to 200 μJ energy in 250 fs (assuming sech² pulse
shape) with good temporal quality leading to ~ 750 MW peak power.

The mode-locking of dissipative soliton fiber lasers using large mode area fiber supporting multiple transverse
modes is studied experimentally and theoretically. Experiments using large core step-index fiber, photonic crystal
fiber, and chirally-coupled core fiber show that when the higher order mode content exceeds -27 dB, the maximum
stable single-pulse energy is significantly reduced. The averaged mode-locking dynamics in a multi-mode fiber are
studied using a distributed model. The co-propagation of multiple transverse modes is governed by a system of
coupled Ginzburg-Landau equations (CGLEs). Simulations show that stable and robust mode-locked pulses can
be produced. The maximum stable single pulse energy is found to increase with higher order mode filtering. This
work demonstrates that mode-locking performance is very sensitive to the presence of multiple waveguide modes
when compared to systems such as amplifiers and continuous-wave lasers, and gives a quantitative estimate of
what constitutes effectively single-mode operation. Robust, distributed higher order mode filtering is necessary
to maximize single-pulsing energy.

Amplified ultrashort pulses at 2 μm are of great interest for atmospheric sensing, medical, and materials processing
applications. We describe the generation and amplification of femtosecond 2 μm pulses in thulium doped silica fiber.
Mode-locked eye-safe laser pulses at ~2 μm were generated in a Tm:fiber oscillator using a single-walled carbon
nanotube saturable absorber. Stable mode-locking was achieved at a repetition rate of 70 MHz with soliton pulses
reaching energies of ~40 pJ with a spectral bandwidth of ~8 nm. Autocorrelation measurements indicated bandwidth
limited pulses of ~500 fs duration. This oscillator was used to seed a Tm:fiber amplifier in both free space and fiber
coupled configurations. Effects of dispersion compensation and pulse amplification are reported.

Non-adiabatic pulse compression of cascaded higher-order optical soliton is investigated. We demonstrate high degree
compression of pulses with soliton order N=2, 3, 4 and 5 in two or three nonlinear fibers with different second-order
dispersion coefficients. Each fiber length is shorter than half of its soliton period. This compression technique has
significant advantages over the widely reported adiabatic and higher-order soliton compression.

For the first time, a non-destructive technique for spatially resolving the location and relative concentration of rare-earth
dopants in an optical fiber is demonstrated. This novel technique is based on computerized tomographic detection of
spontaneous emission and achieves micron-scale spatial resolution with the aid of oil-immersion imaging. In addition to
elucidating interactions between the signal, pump, and dopant distributions, the measurement described here can reveal
shortcomings in fiber manufacturing. Since the technique is non-destructive and can be scanned along the fiber length, it
can map the full 3-dimensional distribution of complex rare-earth-doped fiber structures including gratings, physical
tapers, fusion splices, and even couplers. Experimental data obtained from commercially available Yb-doped silica
optical fibers is presented, contrasted, and compared to refractive index profile data. In principle the technique can also
be applied to Er-, Bi-, or Tm-doped silica or non-silica optical fibers.

We report the energy scaling of mode-locked fiber lasers using a large-mode area chirally-coupled core fiber. This is a
demonstration of the scaling of ultrafast fiber oscillators to large cores in an all-solid glass package that holds the lowest
order fiber mode while maintaining compatibility with fiber fusion technology. An all-normal dispersion cavity design
yields pulse energies above 40 nJ that dechirp to durations below 200 fs. Using lower net dispersion, pulses dechirping
close to 100 fs are obtained with pump limited energies. Effectively single-mode operation is confirmed by beam quality
as well as spectral interference measurements.

The properties of a large mode area Yb-doped double-cladding hybrid photonic crystal fiber, with antisymmetric
high-index inclusions, have been analyzed. Simulations, carried out through a finite-element based modal solver,
and experimental measurements have demonstrated the narrow spectral filtering capability of this fiber, with a
passband of about 80 nm in the Yb gain region. Moreover, high-order mode suppression has been demonstrated
with a proper air-hole size. Finally, the stress-induced birefringence due to the Ge-doped rods used as high-index
insets has been investigated, accounting for the polarization-maintaining behaviour of the manufactured fiber.

We report on a fast measurement procedure for the widespread beam propagation ratio of light emerging from
LMA fibers. The investigated beam is decomposed into its eigenmodes using a computer generated holographic
filter. The modally resolved measurement of amplitudes and phases enables the reconstruction of the optical
field. With the full field information, the propagation of the beam through free space is simulated and a virtual
caustic measurement is realized. After comparing the presented method with ISO-standard measurements it is
applied to conventional step-index fibers as well as to a multicore fiber.

Enabling Single-Mode (SM) operation in Large-Mode-Area (LMA) fiber amplifiers and lasers is critical, since a SM
output ensures high beam quality and excellent pointing stability. In this paper, we demonstrate and test a new design
approach for achieving ultra-low NA SM rod fibers by using a spatially Distributed Mode Filter (DMF). This approach
achieves SM performance in a short and straight rod fiber and allows preform tolerances to be compensated during draw.
A low-NA SM rod fiber amplifier having a mode field diameter of ~60μm at 1064nm and a pump absorption of 27dB/m
at 976nm is demonstrated.

We investigate experimentally and theoretically the wavelength dependence of the pump absorption along Yb3+-doped
fibers, for cladding-pumped single as well as coupled multimode (GTWaveTM) fibers. We show that significant spectral
absorption distortions occur along the length with the 976nm absorption peak affected the most. We have developed a
novel theoretical approach, based on coupled mode theory, to explain the observed effects. We have also investigated the
mode mixing requirements in order to improve the absorption spectral distribution along the increase the overall
absorption efficiency and discuss the implications on fiber laser performance.

Sintering of Yb-doped fused silica granulates is a well established technique developed by the IPHT and Heraeus
Quarzglas and it produces very homogeneous rare earth doped bulk silica core rods for fiber laser applications. By using
a newly developed laser induced deflection (LID) technique we are able to pre-characterize directly the material
absorption properties of the bulk material prior to the laser fiber production. The bulk absorption results measured by
LID are without scattering effects and they are typically in good agreement with the total attenuation measured in the
fibers. We achieved a fiber background loss of 20 dB/km. Furthermore, we present detailed studies of the refractive
index homogeneity of the Yb-doped bulk materials and laser fibers to show the unique features of the Yb-doped bulk
silica.
Multimode double cladding laser fibers with an extra large mode area XLMA fiber design (core diameter up to 100 μm)
have been produced from the Yb-doped bulk silica rods by two different techniques. One is a classical jacketing method;
the other employs the stacking of un-doped, Yb- and F-doped rods and F-doped tubes.
Different fiber types have been tested in different fiber laser setups. The influence of the fiber end cap properties on the
fiber laser focus shift is discussed in detail. We have achieved fiber laser output powers up to 1.925 kW, limited only by
the pump power. We also investigated the long term laser stability at different power levels.

The single-mode regime of 19-cell Yb-doped double-cladding photonic crystal fibers, successfully exploited for
high-power applications due to their large mode area, has been studied. The first higher-order mode cut-off
wavelength has been evaluated taking into account the crossing between its dispersion curve, obtained with a
full-vector modal solver based on the finite element method, and the one of the fundamental space-filling mode,
calculated for an infinite cladding. Moreover, the overlap integral on the doped core of the higher-order mode
at cut-off condition has been calculated, in order to investigate its effective suppression in the gain competition
with the fundamental mode, by applying a spatial and spectral amplifier model. 19-cell double-cladding photonic
crystal fibers with different core diameter and refractive index values have been considered. Simulation results
have shown that the approach based on the fundamental space-filling mode effective index is not suitable for the
cut-off analysis of large core fibers with a finite cladding dimension.

We demonstrate all-fiber passively mode-locked optical frequency comb source at 1.55 μm stabilized to the absorption
line of the hydrogen cyanide. With reduced cavity length 169 MHz fundamental repetition frequency, 111 fs pulse
duration and 44 nm wide optical comb were obtained. Frequency of the optical comb was tied to the frequency stabilized
DFB laser diode Absolute stability of the single comb line was measured to be 9e-9. We expect that stability of 1e-10
within the whole comb bandwidth can be obtained with proposed configuration.

In this work, we have constructed an actively Q-switched T-DCF laser with an acousto-optic modulator (AOM).
Actively Q-switched laser with T-DCF generated stable emission at 1065 nm over a wide repetition rate range of 0 Hz -
150 kHz, with the highest measured pulse energy of 1.6mJ limited by stimulated Brillouin scattering (SBS). We attribute
the robust operation at low duty cycles to the intrinsically low amplified spontaneous emission (ASE), provided by the
tapered fiber shape.
In addition to the advantage of the large mode area at the output end of the fiber, the T-DCF provides several other
attractive features. First, the vignetting of co-propagating ASE results in ASE power loss in wide-to-narrow end
propagation. Second, the low-mode nature of counter-propagating spontaneous emission, arising from mode selection in
the narrow part of the fiber, leads to weaker amplification of the spontaneous emission. These two effects contribute to
the low ASE background. Finally, fiber diameter modulation is a known method for SBS suppression, and another
inherent property of the T-DCF. These characteristics allowed for generation of 1.6mJ, 64 ns pulses at very low duty
cycles, up to single shot operation, illustrating the potential of the T-DCF for high-energy pulse generation and
amplification.

A new measurement technique for characterizing the magnitude of power coupling from the fundamental mode to higher
order core- and cladding-bounded modes occurring in a fiber Bragg grating (FBG) inscribed in a large-mode-area (LMA)
fiber is demonstrated and studied. The method is based on inducing mode selective fiber bending losses on the modes
propagating in the core and monitoring the power guided by the cladding of the LMA fiber. Besides transmitted, also
reflected distributions of modes can be resolved in terms of the relative powers carried by them and thus the fraction of
higher order modes (HOMs) can be quantified. Additionally, the method can distinguish the mode content spectrally
with high resolution. Sample FBGs having a chirped index profile are characterized using the method. It is shown that
the method can yield information that is useful for better design and optimization of fiber optic devices utilizing FBGs in
LMA fibers, such as fiber lasers.

Increasing the ablation efficiency of picosecond laser sources can be performed by bunching pulses in bursts1 and benefit
from heat accumulation effects2-5 in the target. Pulsed fiber lasers are well suited for such a regime of operation, as the
single pulse energy in a fiber is limited by the onset of nonlinear effects (SPM, SRS). Increasing the number of pulses to
form a burst of pulses allows for average power scaling of picosecond fiber lasers. We are presenting in this paper a
high-power fiber laser emitting arbitrarily-shaped bursts of picosecond pulses at 20 W of average output power. Burst
duration can be varied from 2.5 ns to 80 ns. The burst repetition rate is externally triggered and can be varied from 100
kHz to 1 MHz. The single pulse duration is 60 ps and the repetition rate within a burst is 1.8 GHz. The output beam is
linearly polarized (PER > 20 dB) and its M2 value is smaller than 1.15. The laser source has a tunable central wavelength
around 1064 nm and a spectral linewidth compatible with frequency conversion. Conversion efficiency higher than 60%
has been obtained at 10 W of 1064-nm output power.

Optical fibers are used in various applications, e. g. optical communication, material processing, as a laser medium
or to generate efficient supercontinua. For most of these applications the knowledge of the dispersion is an
essential prerequisite. The dispersion and modal properties of photonic crystal fibers (PCF) strongly depend
on the hole diameter and pitch. Since fabrication tolerances affect the structure of the photonic lattice, the
dispersion behavior as well as the number of guided transverse modes can differ from numerical calculations.
Dispersion measurement of singlemode photonic crystal fibers has been well described in recent papers. However,
the determination of dispersion in the presence of higher-order modes is much more difficult.
To measure the dispersion of optical fibers with high accuracy, a time-domain white-light interferometer based
on a Mach-Zehnder interferometer is presented. The experimental setup allows to determine the wavelength-dependent
differential group delay of light travelling through conventional fibers and PCFs within the wavelength
range from VIS to NIR. Interferences appear due to superposition of two laser beams, one propagating through
the tested fiber and the other travelling through air. Measuring the different group delays of a step-index fiber
shows the sufficient accuracy of the interferometer.
This paper demonstrates a simple yet effective way to suppress higher-order modes, making it possible to
measure the chromatic dispersion of singlemode as well as multimode fibers.

In 2009, we introduced a new doping concept involving Al2O3/rare-earth nanoparticles (NP) in a MCVD-compatible
process finding potential applications in Erbium-, Ytterbium- or Erbium-Ytterbium-doped fiber
amplifiers and lasers.1 This approach, motivated by the need for increased efficiencies and improved attributes,
is characterized by the ability to control the rare-earth ion environment independently from the core composition.
The NP matrix can therefore be viewed as an optimized sub-micronic amplifying medium for the embedded rareearth
ion. The first experimental evidence to support this idea is reported in a comparative study with a standard
process2 where homogeneous up-conversion (HUC) and pair-induced quenching (PIQ) levels are extracted from
Er3+ unsaturable absorption measurements. NP-based fibers are found to mitigate the effects of the Er3+ concentration increase seen in standard heavily-doped fibers. This conclusion is particularly clear when focusing
on the HUC coefficient evolution since, for a given type of NP, its level is independent from the Er3+ concentration
in the doped zone. In this paper, we address our most recent work completing these preliminary results. First,
we investigate the quenching signature of a new NP design and its behavior when incorporated in different core
matrices. The interplay is further analysed by relating this set of measurements to practical EDFA performances.
Gain and noise characteristics of typical WDM amplifiers operating points serve as key benchmarking indicators
to identify the benefits of NP Erbium-doped fibers in the wide variety of EDFAs implementations.

In this paper, we propose a novel all-fiber laser scheme with the output wavelength of 1565 nm based on a new Er-doped
fiber design. A conventional commercially available pump source at 980nm (D=105μm, NA=0.2) is used for pumping.
A high slope efficiency of 28% comparable with those in Er-Yb lasers was achieved owing to the utilization of the novel
P2O5-Al2O3-SiO2 (PAS) glass as the host for Er3+ ions. A relatively low in-cavity fiber length (~14m) becomes possible
owing to a small outer fiber diameter (80μm) and the use of a fiber taper (105μm to 80μm) for launching the pump.